The present invention relates to an improvement in a thermal transfer recording medium having a release layer between a foundation and a heat-meltable colored ink layer.
Hitherto, in the case of a thermal transfer recording medium of the type capable of forming printed images of satisfactory quality even on a paper sheet of poor surface smoothness (hereinafter referred to as "rough paper"), generally, there has been employed a heat-meltable ink layer exhibiting bridging characteristics (i.e., a heat-meltable ink layer capable of being transferred as bridging over depressed portions of the rough paper). For the purpose of obtaining a heat-meltable ink layer having enhanced bridging characteristics, there is used a heat-meltable vehicle composed mainly of a resin and having a great cohesive force as the vehicle of the heat-meltable ink.
A thermal transfer recording medium used for printing bar codes on labels, and the like, also employs a heat-meltable vehicle composed mainly of a resin and having a high cohesive force because printed images with good fastness are required.
However, when a thermal transfer recording medium having on a foundation such a heat-meltable ink layer of which the vehicle is composed mainly of a resin is brought into contact with a receptor paper, and after being heated with a thermal head, peeled from the receptor paper when printing, a heated portion of the heat-meltable ink layer retains a great adhesion force to the foundation and is hard to come off from the foundation, which results in a poor transfer.
A way of improving the transferability of the ink layer has been adopted wherein a release layer composed mainly of a wax and having a low melt viscosity is interposed between the ink layer and the foundation.
However, the use of such a release layer having a low melt viscosity for improving the releasability of the ink layer from the foundation when printing causes the following problems.
(1) Since the adhesion force between the ink layer and the foundation is weak when not being heated before printing, the phenomenon called "falling of ink like a powder" (hereinafter referred to as "ink-falling") occurs. The ink-falling phenomenon means that the ink layer is peeled off from the foundation in a powdery form even by action of a small physical force such as rubbing. PA1 (2) Since the adhesion force between the release layer and the ink layer is weak, the unheated portion of the ink layer adjacent to a heated portion of the ink layer is peeled from the release layer together with the heated portion to be transferred to a receptor paper, which means that undesired transfer of the ink layer takes place. In the phenomenon, there occurs smearing of the receptor paper with the ink like a tail on the opposite side of the printed image relative to the traveling direction of the thermal head. Thus, the phenomenon is also called "tailing phenomenon". PA1 the heat-meltable release layer comprising particles comprising a wax and having an average particle size of 1 to 6 .mu.m, the coating amount of the particles being from 0.3 to 2.5 g/m.sup.2, PA1 the average particle size and the coating amount of the particles satisfying the relationship represented by formula (I): EQU 0.28R.ltoreq.M.ltoreq.0.83R (I)
When a release layer having a high melt viscosity is used to eliminate the aforesaid drawbacks, the releasability of the ink layer from the foundation when transferring is poor, so that it makes no sense to provide the release layer. Further, since the foundation is subject to a great force when the ink ribbon is peeled from the receptor paper, wrinkles are produced in the foundation to cause an extraordinarily larger diameter of the roll of the ink ribbon wound on the take-up reel in the ribbon cassette to make it difficult to rewind the entire length of the ink ribbon and to cause uneven end surfaces of the roll of the ink ribbon wound on the take-up reel. The wrinkles also cause an unevenness in the travel of the ink ribbon and dropout portions in printed images.
On the other hand, Japanese Unexamined Patent Publication No. 61-268485 discloses a technique wherein a layer composed of a fine powder is provided between the foundation and the ink layer to improve the releasability of the ink layer. However, since the fine powder layer does not contain a heat-meltable binder such as wax, the adhesion force of the layer to the foundation is weak, so that the aforesaid ink-falling phenomenon is likely to occur. If the adhesion force of the powder layer to the foundation is increased, for example, by elevating the drying temperature in coating in order to prevent the ink-falling phenomenon, the releasability of the ink layer from the foundation when transferring is poor.
In view of the foregoing, an object of the present invention is to provide a thermal transfer recording medium wherein the adhesion force of the ink layer to the foundation is strong when not being heated before printing and the releasability of the ink layer from the foundation is good when the ink ribbon is peeled from the receptor paper in printing, thereby causing no problems such as ink-falling phenomenon and tailing phenomenon.
This and other objects of the present invention will become apparent from the description hereinafter.